The invention relates to a method for actuating a braking device in the drive train of a vehicle, especially a motor vehicle, with an automatic transmission according to the preamble of claim 1. Furthermore, the invention relates to a control apparatus for actuating a braking device in the drive train of a vehicle, especially a motor vehicle, with an automatic transmission according to the preamble of claim 24 as well as a vehicle according to claim 25.
In modern motor vehicles with an automatic transmission, various securing mechanisms exist to park the vehicle securely. These components of securing mechanisms forming a braking device are in essence the parking brake and the service brake as well as the parking lock in the automatic transmission itself. The service brake mainly enables the reduction of the vehicle speed up to the standstill of the vehicle and is regularly engaged by means of a brake pedal, but can also be developed by means of a braking force booster device (active braking force booster) or slip control system (ESC), for example in highly automated or piloted driving. The service brake acts preferentially on all wheels of the vehicle. The service brake is often designed as a hydraulic service brake, in which the braking force is transmitted hydraulically by means of fluid pressure in brake lines, but alternatively, it may also be a pneumatic service brake, in which the braking force is transmitted pneumatically by means of compressed air in the brake lines.
The parking brake (often also called emergency brake) is conventionally a mechanic parking brake that is engaged by the driver with the foot or hand and that creates a locking force by means of pull cables on the respective vehicle wheels. Specifically, a mechanical force is created by the operator by means of an actuating unit, which is transmitted by means of mechanic pull cables up to the brake system where then a force is exerted on a brake disc or brake drum. Modern parking brakes are designed as electric or electronic parking brakes where the actuators transmitting the braking force are actuated by means of an electric control apparatus. The parking brake is intended to secure a stationary or parked vehicle against rolling away, even on an inclined roadway. In other words, this means that the parking brake, like the parking lock described in further detail below, is generally designed for continuously stopping (parking) without a time limitation, whereas the service brake is principally designed for short-term stopping.
The parking lock in the automatic transmission works in such a way that when the driver selects the gear “P”, for example by means of a selector lever, a parking pawl in the gearbox is engaged which is preferentially controlled electrically or operated electro-mechanically. This parking pawl engages with the respective park-lock wheel which is preferentially positioned on a drive shaft of the gearbox or of the drive train in a rotationally fixed manner. The parking pawl may be connected to the gearbox housing itself, for example. As the parking pawl engages with the park-lock wheel, the latter is blocked in such a manner that it cannot be rotated anymore, thereby also blocking the driving shaft of the gearbox or drive train, and therefore also the wheels, of the vehicle. When a vehicle is parked on a slope at an incline, for example (or generally speaking, on a sloping plane), the parking lock is conventionally under stress, as the vehicle, after the gear “P” was engaged, will still roll a little backwards until the parking lock engages. Concretely, the park-lock wheel is subjected to a torque, which is determined by way of the downward force on the vehicle. This torque is supported by the pawl on the gearbox housing, which eventually causes the pawl cogs to interlock strongly with the cogs of the parking lock, resulting in a right-sided parking lock charge or a left-sided parking lock charge, depending on the direction of the torque. When disengaging or deactivating the parking lock, which involves the disengagement of the cogs of the parking pawl from the cogs of the park-lock wheel, this charged condition in the drive train or gearbox leads to the sudden relaxation of the drive train, which is felt on the vehicle as a distinct jolt in addition to an often loud noise (disengagement impact), which is perceived by a user as unpleasant and disruptive.
An additional disadvantage is that the necessary force to disengage the parking lock increases with the magnitude of the charge. Under certain circumstances, this may lead to the situation that the parking lock can only be disengaged with much force. The magnitude of the charge itself depends on the braking torque of the wheels, which in turn depends on the downward force. The downward force in turn depends on the roadway inclination and on the vehicle weight.
From DE 10 2013 212 829 A1, a system and method are known for combined control of an electronically controlled parking brake and an electronically controlled parking lock of a motor vehicle, embodied as a mechanical locking device, by means of which a disengagement impact is to be prevented when disengaging a parking lock of a motor vehicle designed as a mechanical lock. Specifically, this system comprises an electronic parking brake device for electronically controlling the parking brake, an electronic locking device for electronically controlling the parking lock, an electronic hill start assistance device for electronic control of the parking brake, an electronic device for detecting the presence of at least one parking condition, and an automatic sequence controller, which controls the parking brake device, the locking device and the hill start assistance device in such a way that upon the detection of the parking condition, the parking lock is activated by means of the locking device, the parking brake device starts establishing a tightening pressure sufficient for activating the parking brake, and the parking brake is set by means of the hill start assistance device until the tightening pressure necessary for activating the parking brake has been built up by means of the parking brake device. By activating the parking brake by means of the hill start assistance device until the tightening pressure necessary for activating the parking brake has been built up by means of the parking brake device, a vehicle is to be prevented from moving from a standstill after the activation of the parking lock, when the built-up tightening pressure necessary for activating the parking brake by means of the parking brake device is not yet sufficient for activating the parking brake. This is meant to prevent the charging of the parking brake embodied as a mechanical locking device. This, in turn, is meant to prevent the adverse disengagement impact upon the release of the parking lock. Specifically, the vehicle is thus secured against rolling away solely by the parking brake.
However, despite this measure, the driver may get into situations that generate an undiminished disengagement impact. This happens, for example, when the driver, starting from a stationary position with engaged parking lock, releases the parking brake before disengaging the parking lock, because it allows the vehicle to roll away and fall into the parking lock, which braces itself as described above.